The present invention relates generally to systems for detecting fluctuations of height positions of the recording medium surface used in an optical recording and reproducing apparatus. More particularly, the invention uses a sub-light beam which is introduced into a peripheral part of a laser light beam used for recording or reproducing. The peripheral part contributes almost nothing to the recording or reproducing, per se, but it does enable a detection of the height fluctuations of a surface on the recording medium.
In general, in an optical recording or reproducing apparatus, an objective lens focuses a laser light beam into a spot on the surface of a recording medium. In order to form a very small spot on the recording medium surface, a very small numerical aperture lens is used for the objective lens. The depth of field of focus decreases with an increasing of the numerical aperture. Consequently, the depth of field of a lens used for the above mentioned objective lens is very shallow.
A displacement or fluctuation may occur in the height position of the surface on a rotating recording medium because it is warped and non-flat. Thus, the distance between the objective lens and the recording medium surface fluctuates. However, the depth of field of an objective lens used in an optical recording or reproducing apparatus of this character is particularly shallow. For this reason, if the distance between the objective lens and the recording medium surface fluctuates, even by a slight amount, the diameter of the spot of the laser beam focused on the recording medium surface will fluctuate greatly. In this case, it is often difficult or impossible to have optical recording or reproducing with a good signal-to-noise ratio and high fidelity. Accordingly, it is necessary to positively and accurately hold a constant distance between the objective lens and the recording medium surface.
Heretofore, an optical recording or reproducing apparatus of this character has detected a height position displacement of the recording medium surface and controlled the height position of the objective lens in response thereto, in order to maintain a constant distance between the objective lens and the recording medium surface. This control system is generally referred to as the "focus-servo" system.
The light path of a laser light beam for recording or reproducing is hereinafter called a "main beam", and a light beam for detecting the surface height is hereinafter called a "sub-beam".
A conventional system for detecting the height of the recording medium surface in a known focus-servo system comprises means for projecting, from outside the main beam, and sub-beam having a diameter which is in the same order as the diameter of the main beam. The sub-beam is projected into the objective lens in a slightly oblique direction. The sub-beam is thus reflected by the recording medium so that it passes again through the objective lens, and is directed out of the light path of the main beam by a half mirror. In this system, however, the sub-beam has had beam diameter which is in the same order as the diameter of the main beam. Moreover, the deviation of the sub-beam optical axis is only slight relative to the main beam optical axis. For these reasons, the half mirror is disposed in a position to transverse the entire light path of the main beam.
Consequently, the half mirror, in this known detection system, causes a high loss of light signal strength in the main beam. The rate of light utilization of the main beam decreases.
In another known detection system, a sub-beam is projected from outside the path of the main beam in a slightly oblique direction. The sub-beam is reflected by the recording medium and is again directed into the same light path that was followed by the incident sub-beam, the sub-beam thus returning being detected. In this method, however, it is difficult to cause the sub-beam to follow exactly the same light path in both the forward (incident) path and the return (reflection) path. The adjustment and setting of the detection device is very difficult. As a result, it almost impossible to reduce this system to practice.